Telephone system



G. HECHT TELEPHONE SYSTEM Filed Oct. 25, 1940 Oct. 28, 1941.

Patented Oct. 28, 1941 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM George Hecht, Astoria, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a. corporation of New York Application october 23, 1940, serial No. 362,347

Claims.

This invention relates to telephone systems and particularly to the receiving equipment of a transmission line over which alternating currents, preferably within the voice frequency range, are transmitted for effecting the operation of said equipment to control, in turn, the selective positioning of switching apparatus. More particularly, the invention is in the nature of an improvement over the enabler circuit for sensitizing the detector channels of the receiving equipment shown and described in my copending application Serial No. 362,346, led Oct. 23, 1940. In said application is disclosed a telephone system including a plurality of receiving channels each of which is adapted to respond to a particular frequency in an incoming signal composed of several frequencies, and an enabler circuit therefor which is adapted to sensitize the detector elements of said channels for response to the signal some time after its arrival in order to avoid possible false operation ofthe channels from the-transient disturbance Aset up` by the initial impact of the signal energy upon the filtering devices immediately preceding the detector elements ofthe channels. Since the duration .of the disturbance depends upon the intensity of the signal which, in turn, depends uponlthe length of the loop, the operation of the enabler circuit disclosed and claimed in said copending application depends upon means variably-responsive to the breaking down of a gaseous conductor device coupled to the line and resDOnding to the intensity of the signal, the time when the enabler sensitizes the detector ele.l

ments of the several channels being made contingent upon the quantity of current drawn through the anode of the gaseous conductor device which depends, in turn, upon the difference of; potential across the line set up by the 'signal and, therefore, upon the length of the line itself. 4In the preferred embodiment of the present invention the gas-filled conductor device is eliminated and -the signal to the enabler is made contingent upon a marking potential, determined by a direct current flowing in theloop, being applied to the grid of a vacuum tube Aelectronic device, the current in the cathode-anode circuit of which is correspondingly alteredl to function relay means that sensitize the detec# tors on a time delay basis which corresponds to the quantity of the ,cathode-anode current which, assaid before, is varied, through the potential applied to the grid, in accordane with the length of the loop. i

This .and other features ofthe `invention will b e more readily ascertained from the following detailed description of said invention, appended claims and attached drawing in which is shown schematically the elements of the telephone system completely disclosed in my above-mentioned copending application and certain other elements to which my invention more directly relates.

Referring, now, to the drawing, A represents a subscribers station in an automatic telephone system, said station being equipped with a .de-` vice I suitably arranged to produce one or more currents of different frequencies in the voice frequency range in response to the depression of keys or the like that represent digits or other characters making up the wanted designation, 2 represents a line extending from the station instrument to the central ofiice and established therebetween by any suitable means such as, for instance, by line extending switches and the like. The conductors ofA said line extend to battery and ground serially through the upper and lower windings of relays 4Il|4 and 403, respectively, and resistances 28 and 2l, respectively, with rectifier 442 bridged across the upper winding of relay 403 and rectier 443 bridged across the lower winding of relay 403. The line, for the purpose of switchingY control and further eXtension,-is connected to a free sender through' one of a plurality of selectable sender connector relays I3, said sender comprising coupling transformers 3, 4, 5, 6 and l, amplifiers 8 and 9, volume limiter II, and the enabler circuit I0, the last function-y ing to sensitize the channel detectors fl, f2, f3,`

f4 and f5 which are individually so designated in accordance with the frequency to which each of the channels is` respectively assumed adapted to respond, which channels include individually responding relays, and the settable register relays which follow the operation of the channel relays when operated to signalize an incoming impulse. The amplifiers 8 and 9 and the volume limiter II Iare commercially available circuit units while the channels fI-f5 are completely disclosed in Patent 2,237,742, issued to A. A. Lundstrom on April 8, 1941,` although it is obvious and ltherefore understood that the invention is not limited to channelsv of this description. The enabler I0 comprises the vacuum tube 32, the varistor bridge I6 and the polarized relays II, I9 and 20 with their intermediate capacitorresistance networks controlling the operating times of said Yrelays in the manner described hereinafter.

n the operation of the invention, a call-is initiated at station A by the removal of the station instrument from the switchhook whereupon the line is then extended by any suitable means to the central ofce at which an available sender is then connected to said line through the medium of a sender connector I3 after which key tone is transmitted from equipment, not shown, back to the calling station A as a signal that the sending device I may be operated to transmit the number of the wanted station, all in the manner described in the above-mentioned patent to A. A. Lundstrom.

When any one of the ten keys of the sending device I is operated, two out ofthe five defined frequencies f-f are generated in the combina-1 tion required to designatek the digit representedby the key depressed, and the resulting current producedy thereby is transmitted over a circuit contact of relay I2, over the loop, the top back contact of relay I2, the upper windings of relays 40| and 403, respectively, and through resistance 28 to negative battery. The current flowing through this loop is, of course, determined by its ohmic resistance, the only Variable element of which is the resistance of the loop proper. Since the grid of tube 32 is connected to the line terminal side ofresistance 28 a positive potential is applied thereto which determines the current that will ow through the cathode-anode circuit i of the tube 32, which circuit may be traced from ground through the middle winding of relay I9, `over the yinternal cathode-anode path of the tube,

thyrite element I8, to negative battery. The

".fvaluefof the direct current potential which is apwhich may be traced from one side of the send- Y ing device I, over the lower conductor of the looped pair 2, the lower back contacts of relay I2,'the next to the inner contacts of sender conhector cut-in relay I3, the primary winding of transformer 3, the inner contacts of relay I3, the top backcontacts of relay I2 and the upper conductor of looped pair 2 to the other side of` the `stationsending device I. The current composed of the two frequencies is induced in the secondary winding of transformer 3 and fed into the input side ofamplier 8 whence, after suitable amplification, itis applied, through transformer 4, to thevolume limiter II and, through transformer 1. at a suitable level, to the input pad I4 from which, after passing through suitable lters (not shown). it is available for operating the channel'v detectors fI-j5 when the detector elementsof the'fsame have been sensitized by the enabler I0 as explained hereinafter. 3

Considering that portion of the signal energy which passes into the volume limiter II,F the energy causes the volume limiter to operate and regulate the energy output in accordance with its adjustment which may be such as to limit the final magnitude of output to thatrequired for the satisfactory operation ofv the channel detectors and nov more regardless of the magnitude of the input energy.` The reason' for doing thisis, ofcourse, to prevent overloading any ofthe channel" detectors which, should it occur in the event of a heavy input signal due,.say, to a very' short -loop 2, would be productive ofla heavierv side -of resistance 28.V The cathode of the tube is connected to the thyrite impedance element I8 whose function will be shortly described incon` nection with the operation of the enabler.

vIt will be further observed that the middlel winding of relay I9 is Vin series vwith the cathodeanode circuit of the vacuum tube 32. Now when theline loop is established to the sender connector, a direct current circuit is completed therebetween which may be traced from ground, through resistance 2l, the lower windingsyof relays V403 and 48I, respectivelm the lower back s, plied to the grid prior to the transmission of any of the signal impulses is practically unaffected by the alternating current signal potential superimposed thereon when the signal impulses are transmitted inasmuch as the former is preponderately large in comparison to the unamplied signal voltage which is similarly superimposed upon thegrid, I-Ience the Value of the direct current potential applied to the grid is substantially the only potential which determines the quantity of current that will flow through the middle winding of relay I9. On a zero loop, for example, the direct current Vilowing through the loop is a maximum, the positive potential applied to the grid is a maximum and the current flowing throughthe cathode-anode circuit and, therefore', through the middle winding of relay I9 yisalso a maximum. On a maximum loop, on the other hand, the loop current is a minimum, the positive potential applied to the grid isa -minimum and, therefore, the current flowing through Ythe middle winding of the relay I9 is a minimum.

anode circuit of'tube 32, it becomes necessary to guard against any false variation of this current which may be due to the voltagedrop in the battery 34 as a result. of the current drawn therefrom through the cathode-anode circuit in con- To property of varying its resistance with the quan-v tity of current flowing therethrough, tends to keep the -voltage drop constant so that any variationin the current flowing through the cathodeanode circuitewill be due only to the value of the v positive potential on the grid of the tube 32 which,Y in turn, `will depend'upon the resistance or length ofxthe line'loop. f

The current flowing through the middle windingf of `relay I9, regardless'fof Whether it is a maximum or minimum or any value in between, is in opposition to the current flowing through the circuit permanently established through the right winding ofthegrelay and aiding to the current' flowing inthe circuitrlnormally established from ,ground over the right'contact of relay-20, the right contact of relay I1, resistance 23,' and the left'winding of. relay I9 to battery. As a re- -sult of the currents owing through the three windings priorto' the reception of any signal impulse, fthe-armature of relay I9 assumes thev position shown in the drawing; that is, the armature makes with its'pleft contactjthus keeping condenser 2B short-circuited. gonsider,` now, that-a signalimpulse Since the line loop direct current is to detery minethe current yflowing through the cathode is transmitted. iportionof the signal energy is applied 'throughV transformer 6 and to the Vvaristor bridge I6 whereupon the current is rectified and applied .to theleft winding of relay I'I. This current opposes that which flows through the permanently established circuit through the right winding thereof, causing said relay to operate and remove the normally applied short-circuiting ground to condenser 2| which thereupon begins to charge over the charging circuit extending from positive battery, left winding of relay I9, condenser 2I to ground, and thus temporarily maintain the flow of current through the left Winding of said relay, Which current is normally established through the right contact of relay I1 to `ground on the right contact of relay 20 as already described. When condenser 2| is fully charged,.the current ovv through theleft winding of relay I9 ceases and if current is flowing through fits middle winding as above described f.

relay I9 will release but its time of release is, under the conditions described, determined by the intensity of the current flowing through the middle winding which aids the flux produced throughthe left Winding by the normal circuit therethrough and the charging circuit of condenser 2I. For a minimum loop, when the current ,through the middle winding is a maximum due to the direct current over the loop being a maximum, relay I 9 releases in its maximum time and, in so doing, closes a charging circuit for condenser 26 through the left Winding of relay 20 via resistance 33. The current of the charging circuit opposes that through the circuit of the right winding and operates relay 20 which applies ground to conductor I and over Said conductor to the cathodes of the detector tubes of the channel detectors fI-f5 thereby activating said tubes for response to the signal energy incoming over pad I4. The length of the closure will, of course, depend upon the capacity of condenser 26 which may be chosen for any value desired.

For a maximum loop, when the current through the middle Winding is a minimum, the relay Will release in a minimum time to connect condenser 26 to the left winding of relay 20. Between the extreme of maximum direct current at zero loop when relay I9 operates in the maximum time iixed by the capacity of condenser 2|, and minimum direct current at maximum loop when relay I9 operates in minimum time, the time of operation of said relay will progressively and inversely vary with the intensity of the direct current fixed by the length of the loop.

The length of time during which the armature of relay 2l) engages its left contact to cause the channel detectors to remain activated depends, of course, upon the constant of condenser 26 and resistance 33 Which may be chosen to provide any interval desired. However, when the condenser is completely charged, the current through the right winding of relay again becomes effective, the armature of the relay breaks away from its left contact and ground is removed from conductor I5 and, therefore, from the cathodes of the detector elements of the several detector channels fI-fS which, in consequence, Will no longer respond to any signal energy forthcoming over resistance pad I4 and thereby cause their separate anode relays to release.

When the signal impulse is over (assuming the correct depression of a key at device I), relay I1 will release, reestablishing the short circuit around condenser 2| and the circuit through the left Winding of relay I9 to cause it to make its 75 left contact. Since relay 20 'has already reestablished its armature on its right contact, the circuit is normally inreadiness to receive the next signal when the operations described above are repeated.

While I have described my invention andthe means for utilizing the same in connection with a specic embodiment thereof, it is to be understood that various other embodiments thereof may be made by those skilled in the art without departing lfrom the spirit of the invention as defined Within the scope of the appended claims. Thus, for instance, the middle winding of relay `I9 may Vbe ytapped directly to resistances 28 and 27 Via two pairs of contacts on relay I3, thereby eliminating altogether the necessity of using tube 32. With lappropriate direct current voltages feeding direct current to the line loop, .the potential differences developed across resistances 27 `and 28 by the flovv of the line current could be applied directly to the middle winding of relay I9 whose operating time will be controlled, as before, by the current in the line producing the difference of potential above mentioned.

What is claimed is:

1. In a telephone system in combination,` a transmission line provided with a plurality' of channel detectors adapted for response-to alternating current signals, a source of direct current potential for connection to said line, andan enabler for said channel detectors adapted to sensitize the same for response to said alternating signals comprising means responsive to the difference of potential available across said line in response to the direct current flowing thereover from said source of direct current potential, and means responsive to said incoming signal energy, the speed of operation of said first-mentioned means being dependent upon the value of said potential difference and the response of said second-mentioned means.

2. In a telephone system in combination, a transmission line provided with a plurality of channel detectors adapted for response to alternating current signals, a source of direct current potential connected to said line, and an enabler for sensitizing said channel detectors for response to said alternating current signals at a time subsequent to the arrival of said signals comprising means jointly operative to said alternating current signal and to the difference of potential available across said line in response to the direct current flowing thereover from said source of direct current potential, and other means responsive thereto for sensitizing said detectors, the speed of operation of said first-mentioned means being determined by the value of said potential difference.

3. In a telephone system in combination, a transmission line provided with a' plurality of channel detectors adapted for response to alternating current signals, a source of direct current potential connected to said line, and an enabler for sensitizing said channels for response to said alternating current signals comprising an electronic device provided with a cathode-anode circuit the quantity of current floW through which is dependent upon the difference of potential available across said line consequent to the flow of direct current thereover from said source of potential, means responsive to said alternating current signals, and means jointly responsive to the flow of current in the cathodeanode circuit of said electronic device and to said alternating current responsive means for sensitizing said channel detectors, the speed of operation of s aid jointly responsive means being dependent upon the quantity of current owing in the cathode-anode circuit of said electronic de-A vice.

4. In a telephone system in combination, a transmission line provided with a plurality of channel detectors adapted for response to alternating current signals, a source of direct current potential applied to said'line, and an enabler for sensitizing said channel detectors for response to said alternating current signals comprising an electronic device having al grid connected to said line and deriving a potential through the direct current iiowing over said line from said source of direct current potential, a polarized relay included in the cathode-anode circuit of said electronic device for sensitizing said channel detec tors when operated by the current flowing through said cathode-anode circuit, and means controlled by said incoming alternating current signal for delaying the operation of said relay for an interval contingent upon the intensity of the current flowing through said cathode-anode circuit.

5. In a telephone system in combination, a transmission line provided with a plurality of channel detectors adapted for response to alternating current signals, a source of direct cur;-

rent potential applied to said line, and an enabler connected to said line and deriving a potentialthrough the direct current flowing over said `line from said source of direct currentpotential, a polarized relay having two windings of which one is included in the cathode-anode circuit of said electronic device, means including a second relay and an impedance network operative in response to an incoming alternating current signal for controlling a current flow through the second winding of said polarized relay whereby a current flow is maintained therethrough for an interval after the response of4 said means to oppose the operation of said polarized relay on the cath.- ode-anode current flowing through the rst winding thereof, said interval being dependent upon the constants of said impedance network and the intensity of the cathode-anode current, and means responsive to the operation of said polarized relay for sensitizing said channel detectors.

GEORGE HECl-ITV 

